Allocate a Dart_CMessage structure when decoding a message into C structures

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index 0ca7c064a5b3873258272262911f5442ef718945..08a76a3318ea2aa882f31eeb540d4421f8cd85d8 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -1360,7 +1360,7 @@ DART_EXPORT Dart_Handle Dart_SetNativeResolver(
 DART_EXPORT void Dart_InitPprofSupport();
 DART_EXPORT void Dart_GetPprofSymbolInfo(void** buffer, int* buffer_size);
 
-// --- Message encoding/decoding ----
+// --- Message sending/receiving from native code ----
 
 /**
  * A Dart_CObject is used for representing Dart objects as native C
@@ -1392,10 +1392,17 @@ struct Dart_CObject {
 };
 
 /**
- * A Dart_CMessage is used for encoding and decoding messages from native code.
+ * A Dart_CMessage is used for receiving and sending messages from
+ * native code not running in an isolate. A message contains an object
+ * graph represented as Dart_CObject structures rooted as the provided
+ * root.
+ *
+ * For information on the lifetime of this data, when provided in
+ * callbacks, see the documentation for the individual callbacks.
  */
 struct Dart_CMessage {
-  Dart_CObject** message;
+  Dart_CObject* root;
 };
 
+
 #endif  // INCLUDE_DART_API_H_